/Engine/Source/Runtime/Engine/Public/UnrealClient.h
Copies the hit proxies from an area of the screen into a buffer. InRect must be entirely within the viewport's client area. If the hit proxies are not cached, this will call ViewportClient->Draw with a hit-testing canvas.
void GetHitProxyMap ( FIntRect InRect, [TArray](API\Runtime\Core\Containers\TArray)< [HHitProxy](API\Runtime\Engine\HHitProxy) * > & OutMap )